Method for operating a position measuring device and position measuring device suitable therefor

ABSTRACT

A position measuring operation system for determining positions of a first object and a second object, which can be moved in relation to the first object, including a position measuring system having a light source that is operated in a pulsed manner and an evaluation unit connected to the position measuring device and arranged downstream of the position measuring device. The evaluation unit requests transmission of data from the position measuring device by request pulses, which are separated by defined request cycle times and wherein a defined light source cycle time lies between the chronologically sequential light source exciter pulses and the light source cycle time is determined as a function of the request cycle time in such a way that a chronological synchronicity is assured between the light source pulses and the request pulses

[0001] The present invention relates to a method for operating a position measuring system, as well as to a position measuring system suitable for this.

[0002] Known position measuring systems are customarily supplied with a defined operating voltage by the downstream-connected evaluating unit, or control unit, via a connecting cable. The operating or supply voltage must be within predefined tolerance limits for maintaining the correct operation of the position measuring system. As a rule, a defined voltage drop occurs along the connecting cable. So that the voltage drop becomes as small as possible, it is necessary to keep the electrical current consumption, or the power loss, of the position measuring system low. For assuring this it is known to operate the light source of the position measuring system, one of the largest power consumers in the system, not continuously, but in a pulsed manner. In this connection, reference is made, for example, to U.S. Pat. No. 4,079,251; the pulsed light source operation in a battery-buffered incremental position measuring system is disclosed in it. The pulse frequency of the light source, or the clock rate, between the individual light pulses in the proposed system is selected on the basis of an estimate of the maximum output frequency of the generated incremental signals of the position measuring system.

[0003] It is the object of the present invention, a method for operating a position measuring system, which assures the least possible power current intake of the position measuring system during measuring operations.

[0004] It is furthermore an object of the present invention to disclose a suitable position measuring system, by means of which the least possible electrical current consumption of the position measuring system during measuring operations is assured.

[0005] The first partial object is attained by means of a method having the characteristics of claim 1.

[0006] Advantageous embodiments of the method of the invention ensue from the steps listed in the claims depending from claim 1.

[0007] The second partial object listed above is attained by means of a position measuring system having the characteristics of claim 11.

[0008] Advantageous embodiments of the position measuring system of the invention ensue from the steps listed in the claims depending from claim 11.

[0009] In accordance with the invention, the pulsed operation of the light source is matched specifically to the respective evaluation unit, or its interrogation pulse rates. As a rule data, such as absolute position data, are requested in a chronologically fixed clock pattern from the position measuring system via the evaluation unit by means of request pulses. The synchronization of the light source cycle times with the respective request cycle times, or the matching of the light source cycle times to the request cycle times, preferably takes place during an initialization phase prior to the actual measuring operation.

[0010] It was shown to be particularly advantageous if furthermore a system-specific delay time is taken into consideration for the synchronization of the light source cycle times with the respective request cycle times. This delay time is the result of various system-specific influences between the emission of the light pulse by the light source and the actual determination of the respective data in the position measuring system. Based on the additional taking into consideration of such a delay time, a further increased precision of the absolute position determination results. In this case the system-specific delay time can also be determined by measuring techniques in the mentioned initialization phase; however, alternatively it is also possible to preset a fixed value for the system-specific delay time.

[0011] Thus, an optimum adaptation of the pulsed light source operation to the respective evaluation unit, or to the respective configuration consisting of the position measuring system and the evaluation unit results; besides the current-saving operation of the light source, a precise determination of the absolute position is assured by this, if the transmitted data are absolute position data.

[0012] Alternatively to the transmission of absolute position data, the transmission of other data—from the position measuring system to the evaluation unit can also be provided within the scope of the present invention. For example, these can be velocity and/or acceleration data, which are calculated in the position measuring system on the basis of the determined measured values and are processed for transmission.

[0013] Further advantages, as well as details of the present invention, ensue from the subsequent description of an exemplary embodiment by means of the attached drawings.

[0014] Shown are in:

[0015]FIG. 1, a schematic block wiring diagram of an exemplary embodiment of the position measuring system in accordance with the invention, including a downstream-arranged evaluation unit,

[0016]FIGS. 2a to 2 c, respective signal diagrams for explaining the method of the invention,

[0017]FIG. 3, a flow diagram for explaining the method of the invention.

[0018] A block wiring diagram of an exemplary embodiment of the position measuring system 10 in accordance with the invention, together with a downstream-connected evaluation unit 30, is shown schematically in FIG. 1. A connecting line 20 between the position measuring system 10 and the evaluation unit 30 is also only schematically represented in this drawing figure. Communications between the position measuring system 10 and the evaluation unit 30, as well as the electrical current supply of the position measuring system 10, take place via the connecting line 20. A serial, bi-directional data transfer between the position measuring system 10 and the evaluation unit 20 is preferably provided. The connecting line 20 can of course also include a plurality of individual connecting lines. Basically a number of known variations can be considered for the concrete design of the communications and/or supply connection between the position measuring system 10 and the evaluation unit 30.

[0019] In this example, the position measuring system 10 is suitable for the absolute position determination of two objects which are movable with respect to each other, for example for detecting the tool position in a numerically-controlled machine tool; in this case a numerical machine tool control functions as the evaluation unit 20.

[0020] The position measuring system 10, embodied as an optical position measuring system 10 in this example, comprises a scale 12, which is scanned by means of a scanning unit, which can be moved in relation to it. For this purpose, a light source 11, as well as one or several opto-electronic detector elements 13A, 13B are arranged on the side of the scanning unit. The scale 12 has a graduation structure which permits the determination of a defined absolute position on the basis of the resulting scanning signals. The scanned structure can perhaps be a suitable code structure in one or several tracks; however, other known graduation structures, which make possible an absolute position determination, can also alternatively be employed with the scale 12 within the scope of the present invention.

[0021] The position measuring system 10 can be laid out for detecting linear movements, as well as for detecting rotary movements.

[0022] The scanning signals registered by the detector elements 13A, 13B are fed to a signal processing unit 14 in the position measuring system 10, which determines the respective absolute position data POS from the scanning signals. The suitably processed position data POS are transmitted in the form of a serial data word for further processing to the evaluation unit 30 via a downstream-arranged interface unit 15 and the connecting line 20. In actual measuring operations, the transmission of the absolute position data POS takes place here in such a way that the evaluation unit 30 requests the transmission of absolute position data POS from the position measuring system 10 by means of interrogation pulses ATP; thus the actual absolute position is determined in the position measuring system 10 as the result of the interrogation pulse ATP, and the corresponding data word is transmitted to the evaluation unit 30.

[0023] Accordingly, in the present exemplary embodiment, absolute position data POS are transmitted by the position measuring system 10 to the downstream-arranged evaluation unit 30. As already indicated above, the transmitted data can also be other data, which are calculated in the position measuring system 10 on the basis of measured values, a possibility would be perhaps the transmission of calculated velocity and/or acceleration data, etc. However, in what follows, only absolute position data will be addressed in connection with the transmitted data in the course of the explanation of this exemplary embodiment.

[0024] Depending on the respective type of the evaluation unit 30, the request for the absolute position data POS takes place by means of interrogation pulses ATP, which are separated by defined request cycle times Δt_(NC), i.e. in a customary fixed scanning pattern. The request cycle time Δt_(NC) of an evaluation unit embodied as a machine tool control is approximately Δt_(NC)≡125 μsec.

[0025] In accordance with the invention, the light source 11 is not operated continuously, but in a pulsed manner, for reducing the electrical current consumption of the position measuring system 10. The appropriate control of the light source 11 is performed via a schematically indicated light source control unit 16 in the position measuring system 10. For this purpose, the light source control unit 16 is connected with the light source 11, as well as with the interface unit 15, and therefore also with the evaluation unit 30. For the pulsed excitation of the light source 11, it is charged with light source exciter pulses LP in a suitable clock pattern by the light source control unit 16. The chronological spacing between chronologically sequential light source exciter pulses LP will be called the light source cycle time Δt_(LQ) in what follows.

[0026] How the light source cycle time Δt_(LQ) is determined in accordance with the invention as a function of the request cycle time Δt_(NC) of the evaluation unit 30 will now be explained by means of the subsequent FIGS. 2a to 2 c. The goal of the matching of the light source cycle time Δt_(LQ) with the request cycle time Δt_(NC) is the production of a chronological synchronicity between the light source exciter pulses LP and the request pulses ATP. For a defined configuration of the position measuring system 10 and the evaluation unit 30, this matching preferably takes place during a synchronization phase prior to the actual measuring operation.

[0027] A flow diagram is represented in FIG. 3, by means of which the progress of various method steps will be explained in what follows by means of an example in connection with FIGS. 2a to 2 c. FIG. 2a shows the chronological progress of the request pulses ATP, FIG. 2 provides the chronological progress of the light source exciter pulses LP, and FIG. 2c the chronological relative position of the position determination times with respect to the request pulses ATP and the light source exciter pulses LP in the synchronization phase and at the start of the subsequent measuring operations.

[0028] In the represented example, the synchronization phase (step S10), which precedes the actual measuring operations, starts at the time to. Within the first method step S20, at the time t₁ the light source is excited in a pulsed manner for a short time by means of a first light source exciter pulse LP. Because of various signal-delaying effects, the determination of the actual absolute position data does not take place in the position measuring system at the same time as t₁, but only at a later time t₂, as can be seen in FIG. 2c. Thus, a system-specific delay time Δt_(del) lies between the times t₁ and t₂, which is first determined in the method step S20. The delay time Δt_(del) typically lies in the order of magnitude of a few msec and is caused, inter alia, by signal running times and required build-up times of various electronic components.

[0029] In the course of the method step S20 in the represented example, the previously determined delay time Δt_(del) is checked by means of another pulse-like excitation of the light source at the time t₃ and determination of the actual absolute position data at the time t₄; however, basically this is not absolutely required.

[0030] It should also be pointed out at this time that in step S20 of the method of the invention it is not absolutely necessary to determine the delay time by measuring technology means; instead it is possible on the basis of known system build-up times to fixedly predetermine a value for the delay time Δt_(del) from the outside.

[0031] In the subsequent method step S30 the position measuring system receives a first request pulse ATP from the downstream-located evaluation unit at the time t₅. The second request pulse ATP follows at the chronological distance Δt_(NC) at the time t₈. The chronological distance Δt_(NC) between successive request pulses ATP, i.e. the clock pattern of the request pulses, is determined in method step S30. The respective evaluation units customarily operate at a chronological clock pattern during the measuring operation which does not change during the measuring operation.

[0032] The determination of the chronological distance Δt_(NC) between successive request pulses ATP here is performed, for example, with the aid of a high-frequency oscillator module and a counter coupled with it.

[0033] In the case of an intended determination of the system-specific delay time Δt_(del) by means of measuring technology, it is determined in the same way.

[0034] The values Δt_(NC) and Δt_(del) determined, or predetermined, in this way are thereafter stored in a memory of the position measuring system; an appropriate memory is identified with the reference numeral 17 in FIG. 1.

[0035] In the subsequent method step S40, the light source cycle time Δt_(LQ) is selected, or determined, after the determination of the request cycle time Δt_(NC) and the delay time Δt_(del). This means that the light source cycle time is matched to the request pulses ATP, or their clock pattern. In the present example, the system-specific delay time Δt_(del) is also taken into consideration in the course of this matching. This means that following the determination of the light source cycle time Δt_(LQ), in the subsequent operations the light source exciter pulses LP are always generated at such a time that at the time of the request pulse expected thereafter, the determination of the actual absolute position data already takes place in the position measuring system.

[0036] The synchronization phase is terminated with step S50, and the actual measuring operation can follow in accordance with step S60.

[0037] Therefore, during measuring operations a light source exciter pulse LP is already generated in the represented example at the time t₉, i.e. shortly before the request pulse ATP expected at the time t₂₀. At the time t₁₀, at which the expected request pulse ATP then follows, the system has been sufficiently build-up, so that the absolute position determination can take place. The absolute position determination taking place at the time t₁₁ in the position measuring system then is chronologically synchronized with the request pulse ATP. Analogously therewith, in the subsequent measuring operation this takes place at each expected request pulse ATP, etc.

[0038] Various modifications are now also possible within the scope of the invention.

[0039] It would also be possible in principle not to consider the system-specific delay time ΔT_(del) in the course of synchronizing the light source cycle time Δt_(LQ) with the request cycle time Δt_(NC), for example if this value is negligibly small in an appropriate configuration of a position measuring system.

[0040] The synchronization as described above is preferably performed prior to the actual measuring operations; the objects which can be moved in relation to each other should basically be at a stop. If, however, a relative movement inadvertently results during the synchronization phase, a warning signal is generated for indicating this to the respective user. It is possible, for example, to transmit an alarm bit from the position measuring system to the evaluation unit. But basically other types of warning signals are also conceivable, for example acoustic ones, visual ones, etc.

[0041] The above explained synchronization assumes that the chronological clock pattern of the request pulses ATP does not change during the actual measuring operations. It is therefore advantageous to continue to monitor the request cycle times also during the measuring operation and, in case of a possibly changed request cycle time Δt_(NC), to also generate a warning signal to the user. This, too, can be done as an alarm bit transmitted to the evaluation unit; alternatively it is again possible to generate a visual or acoustic warning signal. In this case the measuring operation would have to be interrupted, and the above explained synchronization would have to be performed again.

[0042] Alternatively to the generation of a warning signal it is also possible to generate a light source excitation signal as soon as possible, or immediately, in the course of a stop, in order to generate absolute position data in the position measuring system and to start the synchronization phase again.

[0043] Finally, in conclusion, the possibility should be pointed out for also transmitting other data than the mentioned absolute position data in the described way.

[0044] Thus, a number of embodiment variations exist, besides the explained example, within the scope of the present invention. 

1. A method for operating an optical position measuring system for determining the position of two objects which can be moved in relation to each other, wherein data are transmitted from the position measuring system (10) to a downstream-arranged evaluation unit (30) in that the evaluation unit (10) requests the transmission of data from the position measuring system (10) by means of request pulses (ATP), which are separated by defined request cycle times (Δt_(NC)), the light source (11) of the position measuring system (10) is operated in a pulsed manner, wherein a defined light source cycle time (Δt_(LQ)) lies between the chronologically sequential light source exciter pulses (LP), and the light source cycle time (Δt_(LQ)) is determined as a function of the request cycle time (Δt_(NC)) between the interrogation pulses (ATP) in such a way that a chronological synchronicity is assured between the light source exciter pulses (LP) and the request pulses (ATP).
 2. The method in accordance with claim 1, wherein absolute position data (POS) are transmitted by the position measuring system (1) to the evaluation unit in the form of data.
 3. The method in accordance with claim 2, wherein for synchronizing the light source cycle time (Δt_(LQ)) with the request cycle time (Δt_(NC)), a system-specific delay time (Δt_(del)), which results between a light pulse (LP) and the actual determination of the data in the position measuring system (10), is also being taken into consideration.
 4. The method in accordance with claim 1, wherein synchronization takes place during a synchronization phase prior to the actual measuring operation, during which the objects which can be moved in relation to each other are at a stop.
 5. The method in accordance with claim 4, wherein a warning signal is generated in case of a relative movement between the two objects during the synchronization phase.
 6. The method in accordance with claim 1, wherein the request cycle time (Δt_(NC)) is monitored and a warning signal is generated in case of a changing request cycle time (Δt_(NC)).
 7. The method in accordance with claim 6, wherein the synchronization is again performed after the generation of the warning signal.
 8. The method in accordance with claim 6, wherein an alarm bit is transmitted from the position measuring system (10) to the evaluation unit (30) as the warning signal.
 9. The method in accordance with claim 4, wherein the request cycle time (Δt_(NC)) of the respective evaluation unit (30) is determined in the synchronization phase.
 10. The method in accordance with claim 4, wherein the system-specific delay time (Δt_(del)) is determined in the synchronization phase.
 11. A position measuring system for determining the position of two objects which can be moved in relation to each other, wherein the position measuring system (10) is designed for transmitting data to a downstream-arranged evaluation unit (30) in that the evaluation unit (30) requests the transmission of data from the position measuring system (10) by means of request pulses (ATP), which are separated by defined request cycle times (Δt_(NC)), and the operation of a light source (11) of the position measuring system (10) takes place in a pulsed manner, wherein a defined light source cycle time (Δt_(LQ)) lies between the chronologically sequential light source exciter pulses (LP), and the light source cycle time (Δt_(LQ)) is determined as a function of the request cycle time (Δt_(NC)) in such a way that a chronological synchronicity is assured between the light source pulses (LP) and the request pulses (ATP).
 12. The system in accordance with claim 11, wherein the position measuring system (10) is embodied for the transmission of absolute position data (POS) as data to the evaluation unit (30).
 13. The system in accordance with claim 12, wherein furthermore a system-specific delay time (Δt_(del)) is taken into consideration for the synchronization of the light source cycle time (Δt_(LP)) with the request cycle time (Δt_(NC)) which results between a light source exciter pulse (LP) and the actual determination of the data.
 14. The system in accordance with claim 13, wherein the request cycle time (Δt_(NC)) of the respective evaluation unit (30) and the system-specific delay time (Δt_(del)) are stored in a memory (17) of the position measuring system. 